Why do games on an external HDMI monitor give higher FPS?

Question

HDMI

I noticed that my favorite game Eador when playing on a Samsung Q530, i5, nvidia M310 laptop gives almost half the FPS than playing on the same laptop, but on an external HDMI monitor connected. In both cases, the game is full screen and the desktop is on two monitors.
Why is that? I want to play "fast" on the built-in monitor.
Google finds similar questions related to different games, but does not find answers.

Answer 1

I can only speculate that this is related to the "V-sync" setting. I faced a similar issue in Diablo III. But he didn't dig further.